The invention relates to hand tools and, more particularly, to cutting hand tools having a pair of pivotal members.
Pivotal member cutting tools such as shears, scissors, and snips are used to cut a variety of materials. Such tools require proper adjustment of the clearance between the pivotal members in order to insure proper operation of the tool and reduced cutting pressure. Adjustment is initially required during manufacture of the tool and, subsequently, in the field in order to compensate for wear, and to provide for better operation when cutting materials of different thickness. Field adjustment is also required as part of reassembly of the tool after sharpening.
Accurate adjustment is particularly important in heavy-duty compound-action tools, known as "aviation-type" snips, used to cut sheet metal. This is due to the very large forces exerted by the blades upon the material and the stresses to which the blades are subjected. Such tools are therefore provided with heavy large-diameter pivot shafts and a broad blade support area, that is, the area of the blades immediately adjacent to the pivot point.
Prior art aviation-type snips provided a threaded pivot shaft which extended through an untapped hole in one of the blades, and was threaded through a tapped hole in the other blade and secured by a locknut. Initial adjustment of such a tool during manufacture consisted of threading the shaft firmly into the tapped hole until the blades were tightly in contact, followed by a slight reverse threading operation of the shaft. A worker would then operate the tool in a normal manner to sense the "feel" of such operation. In a series of iterative procedures, the operator would alternately tighten or loosen the threaded shaft until the "feel" of the tool was correct, in his skilled judgment.
The locknut was then tightened on the shaft to preserve the adjustment. Such initial clearance adjustment was tedious and time-consuming, especially for aviation-type snips having an offset cutting line.
In order to insure proper operation of prior art tools, the tapping of the hole in the tool blade was required to be done with great precision. If the alignment of the tap in producing the threads in the hole was not correct, then blade misalignment would occur. This resulted not only in difficult operation of the tool but in rapid tool wear due to a reduced area of contact between the blades in the blade support area. This, in turn, often led to early failure of the tool.
Certain types of prior art light-duty compound-action tools employed a pair of stamped metal blades connected by a pivot assembly having a threaded shaft passing through untapped holes in each blade and secured by a spring-type lockwasher and a deformed-thread locknut. However, such tools did not readily allow accurate adjustment of blade clearance, and tended to lose adjustment during cutting operations.
In an effort to reduce manufacturing cost, it is desirable to automate as many steps in the manufacturing process as possible. However, the high degree of skill required in the adjustment procedure for prior art tools precluded the use of automation in this step. In addition, the cost of the taps, defective tools due to improperly tapped holes, and warranty costs due to premature failure caused by undetected misaligned tapped holes further added to the cost of such prior art tools.
It is therefore an object of the present invention to provide a cutting tool having a pair of pivotal members, the clearance of which can be quickly and accurately adjusted during manufacture using a process amenable to automation, and which will maintain adjustment when cutting heavy material.
It is an additional object of the invention to provide a cutting tool for which an accurate clearance adjustment procedure can be repeatedly carried out in the field.
It is a further object of the invention to provide a cutting tool having longer life and greater reliability, yet which can be manufactured at lower cost.